In hemocatharsis for therapy of renal failure, etc., blood purifiers such as hemodialyzers, hemofilters and hemodiafilters, comprising dialyzing membranes or ultrafilter membranes as separators, are widely used in order to remove urinal toxic substances and waste products in blood. The dialyzing membranes and the ultrafilter membranes as separators are made of natural materials such as cellulose or the derivatives thereof (e.g., cellulose diacetate, cellulose triacetate, etc.) or synthetic polymers such as polysulfone, polymethyl methacrylate, polyacrylonitrile, etc. The importance of blood purifiers comprising the hollow fiber type selectively permeable separation membranes as separators is very high in the field of dialyzers, in view of the advantages such as the reduction of the amount of extracorporeal circulated blood, high efficiency of removing undesired substances in blood, and high productivity of manufacturing modules.
To use the above blood purifiers as artificial kidney dialyzers, it is needed to completely sterilize the blood purifiers before use. For this sterilization, formalin, ethylene oxide gas, high-pressure steam, or exposure to radioactive rays such as a γ-ray or electron rays is employed, and each of these means exhibits its individual effect. Among those, the sterilization by exposure to radioactive rays or electronic rays is preferably employed because a subject in a package as it is can be directly subjected to a sterilization treatment, and because the sterilization effect of this method is high.
However, it is known that selectively permeable separation membranes for use in such a blood purifier and an adhesive, etc. for use in fixing such membranes tend to deteriorate due to the exposure to radioactive rays or electronic rays. Under such a circumstance, there are proposed methods for sterilization while preventing the deterioration of membranes, adhesives, etc. For example, a method of inhibiting the deterioration of hollow fiber membranes due to exposure to γ-ray by maintaining the hollow fiber membranes in a wet state (cf. Patent Literature 1). However, this method has the following problems: the weight of the blood purifier inevitably increases since it is needed to maintain the hollow fiber membranes in a wet state, which leads to disadvantages in the transport and handling thereof; or the hollow fiber membranes tend to burst or are damaged under such severely cold conditions that the water used to wet the membranes is frozen. Further, the preparation of a large amount of sterilized water is one of factors for higher cost. Furthermore, there is a possible danger of proliferation of bacteria in a very short time interval between the completion of packaging and the starting of sterilization, since the hollow fiber membranes are intentionally maintained in a wet state which facilitates the proliferation of bacteria. As a result, it takes a long time in completely sterilizing the blood purifier thus manufactured, and undesirably, such a disadvantage induces a higher cost and poor safety.
To avoid the wet state of hollow fiber membranes and to inhibit the deterioration thereof due to exposure to radioactive rays, a sterilization-protective agent such as glycerin, polyethylene glycol or the like is contained in the hollow fiber membranes, and such hollow fiber membranes are exposed to γ-ray with their moisture content maintained at not higher than 30% (cf. Patent Literature 2). However, this method suffers from the following problems because of the protective agent contained in the hollow fiber membranes: that is, it is difficult to suppress the moisture content of the hollow fiber membranes lower; the protective agent tends to deteriorate due to the exposure to γ-ray; and it is needed to remove the protective agent by washing the hollow fiber membranes before use.
There is disclosed a method of solving the above-discussed problems (cf. Patent Literature 3). According to this method, hollow fiber membranes of which the moisture content is not higher than 5% are exposed to a radioactive ray under an ambient atmosphere of not higher than 40% RH for their sterilization. This method is effective to solve the foregoing problems and to clear a criterion for the test regulated in the approval standards for manufacturing dialyzer type artificial kidney devices: that is, the UV absorbance (at a wavelength of 220 to 350 nm) of an extract from hollow fiber membranes is lower than 0.1. However, this Patent Literature does not describe or suggest about the following problems: some influences of the ambient atmosphere (oxygen and water) around the hollow fiber membranes (or hollow fiber membrane modules) during the storage thereof act to deteriorate (or oxidize and decompose) the materials of the hollow fiber membranes; and the UV absorbance of the extract (or the amount of eluate) from the hollow fiber membranes tends to increase with time because of the deterioration of the materials of the hollow fiber membranes.
In the meantime, there is disclosed a method of suppressing the insoluble component of the materials of hollow fiber membranes to not higher than 10 wt. % by exposing the hollow fiber membranes to γ-ray with their moisture content kept at not higher than 10 wt. % (cf. Patent Literature 4). It is described in this Patent Literature that the amount of a hydrophilic polymer which is extracted from membranes using a 40% aqueous ethanol solution is not larger than 2.0 mg/m2 per one m2 of the area of the membranes on their sides in contact with a treated fluid.
The present inventors have intensively studied in order to improve the above-described sterilization method by way of exposure to a radioactive ray or an electron ray. As a result, they have found that the sterilization method by way of exposure to a radioactive ray or an electron ray induces the formation of hydrogen peroxide which can not be detected by the above UV absorption spectrometry. As a result of this finding, it is found that a hydrophilic polymer is extracted by the above extraction method. While the mechanism of forming hydrogen peroxide is unknown, the following can be supposed: the deterioration of the base materials of selectively permeable separation membranes is induced by the presence of hydrogen peroxide; hydrogen oxide has an influence on the increase of the amount of an eluate from the membranes, which is detected by the above UV absorbance and which tends to increase after the exposure to the radioactive ray or the electron ray; and the amount of hydrogen peroxide itself tends to increase with time, which further accelerates the deterioration of the materials to thereby increase the amount of the above known extract from the membranes. Accordingly, it is known that strict control is needed for the exposure of hollow fiber membranes to the radioactive ray or the electron ray and for the following storage of the hollow fiber membranes in order to ensure safety as a blood purifier.
In the meantime, Patent Literature 3 and Patent Literature 4 do not refer to the formation of hydrogen peroxide during the storage of hollow fiber membranes and hollow fiber membrane modules, or to an absorbance (or an eluate) which tends to increase with time after the exposure to γ-ray, or to an increase in amount of a hydrophilic polymer (polyvinyl pyrrolidone) in an extract from the membranes using a 40% aqueous ethanol solution. Patent Literature 4 does not refer to the influence of a humidity of an ambient atmosphere around the hollow fiber membranes, on the deterioration of the materials of the hollow fiber membranes.
To avoid the deterioration of the base materials of medical devices attributed to the presence of oxygen, it is known that the medical devices are sealed in packaging media made of oxygen impermeable materials, together with oxygen scavengers, and are then exposed to radioactive rays, and it is also disclosed that this method can be applied to blood purifiers (cf. Patent Literature 5, Patent Literature 6 and Patent Literature 7).
The deterioration of hollow fiber membranes because of the above radiation exposure in the presence of the oxygen scavenger is accompanied by odors (described in Patent Literature 5), a decrease in strength or dialyzing performance of the base materials (described in Patent Literature 6) or a decrease in strength of the base materials or formation of aldehydes (described in Patent Literature 7). However, any of these Patent Literatures does not refer to an increase in amount of the above extracts. Further, any of these Patent Literatures refers to the oxygen concentration in the package under the radiation exposure, but not to the importance of the moisture content of the selectively permeable separation membranes and the humidity of the ambient atmosphere.
Patent Literatures 8 and 9 disclose hollow fiber membrane modules which show decreased amounts of hydrophilic polymers and which use no filling fluid, by displacing the internal atmospheres of the hollow fiber membrane modules with inert gases. However, the oxygen concentrations in atmospheres for the sterilization of the hollow fiber membrane modules are high, and therefore, it is impossible to completely inhibit the deterioration and decomposition of the materials of the hollow fiber membranes under the radiation exposures. Consequently, the amounts of eluates from the hollow fiber membrane modules can not be reduced, and there arises a further problem that the biocompatibility of the membranes becomes poor since the materials of the membranes are crosslinked by the radiation exposures.
Patent Literature 10 discloses a technique of sealing a fluid separation membrane module in a packaging bag. According to this Patent Literature, the fluid separation membrane module and the packaging bag are filled with deairing water before the storage of the fluid separation membrane module in the packaging bag, and the packaging bag is made of a material capable of shutting out an air so as to seal the membrane module. This technique is intended to prevent the fluid separation membranes from partially drying due to the gasification of the air which is caused because of a change in the temperature of the atmosphere during the storage of the fluid separation membranes. However, in this technique, no attention is paid to an increased transport cost attributed to the increased weight of the package or to the proliferation of bacteria during the storage of the membranes.
Patent Literature 1: JP-B-55-23620
Patent Literature 2: JP-A-6-285152
Patent Literature 3: JP-A-2000-288085
Patent Literature 4: JP-A-2001-205057
Patent Literature 5: JP-A-62-74364
Patent Literature 6: JP-A-62-204754
Patent Literature 7: WO98/58842
Patent Literature 8: JP-A-2001-170167
Patent Literature 9: JP-A-2003-245526
Patent Literature 10: JP-A-2004-195380